Stress-corrosion of cold drawn prestressing steels 1. Hot rolled bar 195 725 1300 8.0 53 2. Cold drawn

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  • Stress-corrosion of cold drawn prestressing steels

    J. Toribio Department of Materials Engineering, University of Salamanca, Spain

    Abstract

    This paper evaluates the anisotropic stress-corrosion behaviour of high-strength cold-drawn prestressing steel wires. To this end, two eutectoid steels in the form of a hot rolled bar and cold drawn wire were tested. While a tensile crack in the hot rolled bar always propagates in mode I, in the cold drawn wire an initially mode I crack deviates significantly from its normal mode I growth plane and approaches the wire axis or cold drawing direction, thus producing a mixed mode propagation. In hydrogen-assisted cracking the deviation happens just after the fatigue pre-crack, whereas in localized anodic dissolution the material is able to undergo mode I cracking before the deflection takes place. An explanation of such behaviour can be found in the pearlitic microstructure of the steels. This microstructural arrangement is randomly oriented in the case of the hot rolled bar and markedly oriented in the wire axis direction in the case of the cold drawn wire. Thus both materials behave as composites at the microstructural level and their plated structure (oriented or not) would explain the different behaviour in a corrosive environment. Keywords: stress-corrosion cracking, pearlitic steel, steelmaking, manufacturing, cold drawing, anisotropic fracture behaviour, fracture micromechanisms.

    1 Introduction

    Stress corrosion cracking (SCC) of high-strength prestressing steel is a problem of major technological concern since it can increase the risk of failure of concrete structures. This paper analyses a wide set of SCC laboratory experiments of pearlitic steel wires before and after cold drawing, i.e., the previous (base) hot rolled material and the fully drawn prestressing steel wire (final commercial product used in prestressed concrete, a material of the highest

    © 2005 WIT Press WIT Transactions on Engineering Sciences, Vol 48, www.witpress.com, ISSN 1743-3533 (on-line)

    Simulation of Electrochemical Processes 227

  • interest in structural engineering). A discussion is presented to rationalise the anisotropic SCC behaviour of cold drawn prestressing steel on the basis of its oriented pearlitic microstructure, to formulate micromechanical models of macroscopic SCC behaviour and to provide the engineer with design tools in damage tolerance and structural integrity analyses of prestressed concrete structures.

    2 Materials and effect of cold drawing

    A high strength eutectoid steel supplied from commercial stock by EMESA was used in this work. It is the steel used in the production of cold drawn wire for prestressed concrete. The chemical composition is given in Table 1. This steel was tested in two conditions: firstly, as hot rolled patented cylindrical bars of 12 mm diameter, and secondly, as a commercial 7 mm diameter cold drawn prestressing wire obtained from the bar.

    Table 1: Chemical composition (wt %) of the steel.

    Mo C Mn Si P S Cr Ni 0.74 0.70 0.20 0.016 0.023 0.01 0.01 0.001

    The mechanical properties of both the bar and the wire are presented in Table 2. The fracture toughness KIC was determined using cylindrical pre- cracked specimens obtained from the bar and the wire—for which the plane strain condition is achieved at the inner points of the crack—together with an expression for the maximum stress intensity factor at the deepest point of the crack (assumed semi-elliptical) calculated by using the Finite Element Method (FEM) combined with a Virtual Crack Extension technique.

    Table 2: Mechanical properties of the bar and the wire.

    Steel Young's Modulus (GPa)

    Yield Strength (MPa)

    U.T.S. (MPa)

    Elongation under UTS (%)

    Fracture toughness (MPa m1/2)

    1. Hot rolled bar

    195 725 1300 8.0 53

    2. Cold drawn wire

    190 1500 1830 5.8 84

    The microstructure of both steels consists of fine pearlite with an interlamellar spacing of 0.1 mm. Fig. 1 shows the microstructure of both the hot rolled patented bar and the cold drawn wire in transverse and longitudinal cross sections. While the hot rolled bar has a randomly-oriented microstructure in both transverse and longitudinal sections (Figs. 1a and 1c), the cold drawn wire presents a randomly-oriented appearance in the transverse cross section (Fig. 1b), but a marked orientation in the longitudinal cross section (Fig. 1d), which implies an effect of manufacturing on the resulting microstructure. Thus the cold

    © 2005 WIT Press WIT Transactions on Engineering Sciences, Vol 48, www.witpress.com, ISSN 1743-3533 (on-line)

    228 Simulation of Electrochemical Processes

  • drawn wire presents features consisting mainly of alternate lamellae of ferrite and cementite aligned parallel or quasi-parallel to the wire axis or cold drawing direction. The consequence will be a highly anisotropic behaviour.

    (a) (b)

    (c) (d)

    Figure 1: Microstructure of both the hot rolled and the cold drawn steels in transverse and longitudinal metallographic sections: (a) hot rolled- transverse, (b) cold drawn-transverse, (c) hot rolled-longitudinal, (d) cold drawn-longitudinal.

    3 Stress corrosion tests

    To evaluate the stress-corrosion behaviour of the steels in aggressive media, slow strain rate tests (SSRT) were performed on transversely pre-cracked rods immersed in aqueous environment under electrochemical control. Precracking of the samples was carried out by axial fatigue in air environment, using different fatigue loads during the last step. The maximum stress intensity factor (K)-levels in fatigue were Kmax = 0.28 KIC, 0.45 KIC, 0.60 KIC and 0.80 KIC, where KIC is the fracture toughness of the material in air.

    3.1 Engineering approach: fracture load

    The macroscopic effects of the environment on fracture were quantified through the ratio of the failure load in the solution (critical value FC) to the failure load in air (reference value F0), as depicted in Fig. 2. All results showed the well known anodic and cathodic regimes of environment-sensitive cracking: for higher

    © 2005 WIT Press WIT Transactions on Engineering Sciences, Vol 48, www.witpress.com, ISSN 1743-3533 (on-line)

    Simulation of Electrochemical Processes 229

  • potentials (E = –400 mV SCE) the anodic regime, associated with localized anodic dissolution (LAD); for lower potentials (E = –1200 mV SCE) the cathodic regime, associated with hydrogen assisted cracking (HAC). Since the results do not substantially depend on pH, only the average results for the three pH values used in the tests are shown in the plot of Fig. 2.

    0.5

    0.6

    0.7

    0.8

    0.9

    1

    0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

    HR/HAC HR/LAD CD/HAC CD/LAD

    F c/

    F o

    Kmax/KIC

    Figure 2: Macroscopic results of the slow strain rate tests, quantified through the ratio of the failure load in the solution (critical value FC) to the failure load in air (reference value F0), for both materials (HR: hot rolled, CD: cold drawn) and the two environmental conditions (HAC: hydrogen assisted cracking, LAD: localized anodic dissolution). The plot shows the average results in the tests.

    An important Kmax-effect is observed due to compressive residual stresses in the vicinity of the crack tip during fatigue pre-cracking of the samples. This phenomenon has been discussed in previous works [1], the main conclusion being that high values of Kmax produce strong compressive residual stresses in the vicinity of the crack tip, thus delaying the hydrogen entry (in HAC) or the metal dissolution (in LAD). Both the hot rolled bar and the cold drawn wire are more susceptible to HAC than to LAD (in engineering macroscopic terms). The cold drawing process is beneficial against LAD phenomena, since it clearly increases the fracture load in the anodic regime. However, cold drawing is damaging against HAC processes, since it lowers the fracture load in a hydrogen environment (cathodic regime) for the whole range of Kmax-values.

    3.2 Physical approach: microscopic fracture modes

    Fig. 3 gives the microscopic fracture modes associated with the different materials and environmental conditions. In HAC conditions (cathodic regime) the hot-rolled bar fails in mode I associated with the so called tearing topography surface (TTS) fracture mode [2,3] followed by cleavage-like propagation, whereas the cold drawn wire exhibits a shear topography with some evidence of

    © 2005 WIT Press WIT Transactions on Engineering Sciences, Vol 48, www.witpress.com, ISSN 1743-3533 (on-line)

    230 Simulation of Electrochemical Processes

  • isolated cleavage facets, and the crack approaches the axis direction producing a mixed mode stress state (longitudinal splitting or delamination). There are two embryos of fracture located symmetrically in relation to the initial crack plane (at an angle of about 80º), but only one of them becomes the final fracture path, and thus the initial crack branching progresses along only one of the branches, probably for statistical reasons, which makes it the fracture path of lower fracture resistance or that with the higher concentration of hydrogen.

    (a) (b)

    (c) (d)

    Figure 3: Microscopic modes of fracture for an intermediate Kmax-level of 0.45 KIC: (a) hot rolled bar under HAC conditions s